Mechanical pumps are used in a wide variety of applications for reducing pressure in an enclosed region to approximately 10 milliTorr. However, in some vacuum applications, further reductions in pressure are required. U.S. Pat. No. 4,086,031 to Kuypers, which is assigned to the assignee of the present invention, notes that some analytical instruments, such as gas chromatograph/mass spectrometer instruments, require evacuated environments that exceed the capability of known mechanical pumps. Consequently, a diffusion pump is combined with a mechanical pump to provide a "high-vacuum environment," i.e., a pressure below 10.sup.-4 Torr.
For high vacuum applications, the mechanical pump acts as a "roughing pump" to remove the bulk of the air from a vacuum chamber. A rotating vane, positive displacement pump may be utilized. The diffusion pump then provides further evacuation, with the mechanical pump operating to maintain the backing pressure for the diffusion pump. The conventional diffusion pump operates by use of high speed jets of oil vapor which collide with gas molecules that enter from a vacuum chamber positioned above the diffusion pump. A pool of oil at the bottom of the diffusion pump is rapidly heated to vaporize the oil. A jet of vapor is directed upwardly through a chimney-like assembly and then downwardly and outwardly through one or more nozzles to provide umbrella vapor jets. Air molecules that are struck by the working vapor are pushed downwardly to a region in which the mechanical pump can remove the gas molecules. Thus, the gas molecules are removed by kinetic energy transfer. While the gas molecules are removed, the oil molecules condense on the cooled walls of the diffusion pump. Cooling can be accomplished by directing a flow of air across the exterior walls of the pump or by applying a liquid coolant. The condensed oil molecules are directed back to the pool of oil by gravity return, where re-vaporization allows a continuous cycle of operation.
While oil vapor diffusion pumps have been used to provide low-cost, high-vacuum pumping capability in such applications as scientific instrumentation and production process equipment, these pumps are susceptible to "backstreamed" oil. As previously noted, a diffusion pump may include more than one umbrella, i.e. stage, of supersonic vapor. Oil vapor from the uppermost stage is most susceptible to being undesirably introduced into the vacuum chamber or manifold that is mounted above the diffusion pump.
There are a number of causes of backstreaming. First, collisions between the pumping vapor molecules and the pumped gas molecules invariably project unwanted pumping vapor molecules in an upward direction toward the vacuum chamber or manifold. This effect potentially introduces contamination into the vacuum region and impedes the level of vacuum that can be obtained.
Another major cause of backstreaming is re-evaporation of oil molecules that condense on the walls and/or nozzle surfaces of the diffusion pump, with the re-evaporated molecules being directed toward the vacuum chamber or manifold. While the condensing walls of the pump are cooled to some extent, evaporation does occur.
Backstreaming may also result during startup/shutdown sequences of the diffusion pump system, since the normally supersonic jetstreams collapse and become subsonic. The subsonic streams are more likely to lose molecules to the region that is to be evacuated, than are supersonic streams.
In order to control backstreaming, baffles are mounted between the diffusion pump and the vacuum housing, i.e. the vacuum chamber or manifold. For example, an air-cooled or water-cooled chevron baffle may be introduced between the two components. Such baffles are sometimes referred to as "optically opaque," i.e. a vapor molecule from the diffusion pump is prevented from following a straight-line trajectory into the vacuum housing. Any backstreaming molecules that follow a straight-line trajectory will instead impinge on or collide with a cooled surface of the baffle, condense, and return to the diffusion pump. A chevron baffle is one in which the cross section of a flow path through the baffle has the configuration of a chevron.
What is needed is an apparatus and system that provide an efficient and inexpensive approach to retarding backstreaming from a diffusion pump to a region that is to be evacuated.